1. Field of the Invention
The present invention relates to an optical modulator and, more particularly, to a Mach-Zehnder type optical modulator which allows the interaction appropriately between signal electrodes and an optical waveguide, only by matching phases at input ends of the signal electrodes.
The optical modulation system includes a direct modulation which modulates the intensity of light by superimposing a modulation signal on a driving current of a light-emitting element and an external modulation which stores information in the light by providing an optical component for changing the phase, frequency, strength or polarization of the light outside the light-emitting element. In recent years, research and development on an external optical modulator, having an excellent broad-band property and chirping characteristic, has been considerably made, in response to the need for a high-speed modulation and long distance transmission.
2. Description of the Related Art
As the external optical modulator, there are an electro-optical modulator, a magneto-optical modulator, an acousto-optic modulator, an electric field absorption type modulator and the like. The electro-optical modulator uses the electro-optical effect, the magneto-optical modulator uses the magneto-optical effect, the acousto-optic modulator uses the acousto-optic effect, and the electric field absorption type modulator uses the Franz-Keldysh effect and the quantum-confined Stark effect.
One of the examples of the electro-optical modulator will be explained.
In the electro-optical modulator, an optical waveguide, signal electrodes and earthed electrodes are formed on a substrate having the electro-optical effect. The center part of the optical waveguide is branched into two between two Y-branch waveguides to form first and second waveguide arms, so as to structure a Mach-Zehnder interferometer. The signal electrodes are respectively formed on the two waveguide arms, and the earthed electrodes are formed on the substrate in parallel to the signal electrodes with predetermined intervals therebetween. Light is made incident on the electro-optical modulator to propagate through the optical waveguide, branched into two at a first Y-branch waveguide to propagate through the respective waveguide arms, merged into one again at a second Y-branch waveguide, and outputted from the optical waveguide. When electric signals, for example, high-frequency signals are applied to the respective signal electrodes, refractive indexes of the respective waveguide arms change due to the electro-optical effect, and hence the progression speeds of first light and second light, each of which propagates through the first and the second waveguide arms, change. By providing a predetermined phase difference between the electric signals, the first light and the second light are multiplexed at the second Y-branch waveguide with the different phases, whereby the multiplexed light has a mode which is different from that of the incident light, for example, a high-order mode. The multiplexed light with the different mode cannot propagate through the optical waveguide, and hence the intensity of the light is modulated. The Mach-Zehnder type optical modulator (hereinafter abbreviated to the “MZ optical modulator”) realizes the modulation by the process of the electric signal→the change of the refractive index→the change of the phase →the change of the intensity. The electro-optical modulator like the above is disclosed in, for example, Japanese Unexamined Patent Application Publication No. Hei 2-196212.
The electro-optical modulator like the above which controls the phases of the first light and the second light independently by the respective signal electrodes is particularly called as a Dual-Drive optical modulator (hereinafter abbreviated to “DD optical modulator”).
It should be mentioned that the phases of the lights to be multiplexed in the second Y-branch waveguide correspond to the relationship between the phase of the electric signal and the phase of the light at an interaction start point at which the electric signal and the light start the interaction. Hence, in order to obtain the predetermined phase difference between the phase of the first light and the phase of the second light in the second Y-branch waveguide, it is necessary to supply electric signals correlating to the respective signal electrodes, by adjusting the phases of the respective electric signals to the predetermined phases. Conventionally, the phases of the respective electric signals are adjusted by using a phase compensator which is provided outside, because a reference point for the phase adjustment is not provided in the optical modulator.
It should be noted that, in this method of using the phase compensator, there is a disadvantage that the phase compensator needs to be adjusted for each product. Particularly, when the phase is compensated by the cable length, there is a disadvantage that the deviation is caused after the adjustment according to the temperature change, due to the temperature coefficient. Moreover, the adjustment becomes more difficult as the frequency of the electric signal becomes higher, and when a plurality of the electro-optical modulators are used through the cascade connection, it is necessary to adjust the phases of the respective electric signals to be supplied to the respective electric-optical modulators, which makes the adjustment more difficult.